This invention relates to networks and, more particularly, to network path selections, or identifications.
Many real-world applications can be viewed to comprise a network that includes nodes that are interconnected by links where, typically, each link has a cost that is associated with traversing the link. The costs of traversing a link in the two directions need not be the same. The typical objective is to identify a path from one node of the network (source node, s) to another node of the network (destination or terminus node, t) that is a least-cost path. This objective is usually achieved by representing the network with a mathematical entity known as a directed graph, or digraph, where nodes, or vertices, of a digraph represent the network links, and edges of the digraph that interconnect the nodes represent network links. The least-cost path problem is solved mathematically on the digraph by employing principles that have been developed by digraphs generally, and the solution is then applied to the network.
The application is, of course, dependent on the network. To give one example, the application may be communications, in which case the network may be the telecommunication network, and the least-cost problem may be the problem of identifying a least-cost route for a call or a plurality of calls. Applying the solution to the network is also dependent on the particular application. It may be providing information for the process of establishing a connection along the least-cost route, re-routing a whole set of existing connections in response to a change in the network because of a failure at some node, or it may be just storing the path information for potential connections that are high priority connections. To give another example, the application may be transporting goods over the highways, in which case the network is the network of highways, the least cost problem often is the problem of identifying the least cost route for moving the goods from city A to city B. In this case, applying the solution to the network may be embodied in the process of giving instructions to a driver to execute the movement of the goods from city A to city B.
The classic algorithm for single-source shortest-path in a digraph with non-negative edge weights is the Dijkstra algorithm. For the above-mentioned goods movement application, for example, the vertices of the digraph graph represent cities, the edge weights represent the driving distances between pairs of cities connected by a direct road, and the Dijkstra's algorithm finds the shortest route between any two cities.
The input of the Dijkstra algorithm consists of a weighted directed graph G and a source vertex s in G. The vertices of G form a set V, and the edges of the graph G form a set E. The algorithm computes the least costly path from the source vertex to each of the other vertices in V. In one implementation, the algorithm maintains two sets of vertices S and Q. Set S contains all vertices for which the least cost path is already known (initially, empty), and set Q contains all other vertices (initially, the entire set of vertices, V). The algorithm also maintains an array d of best estimates of shortest path to each vertex, and an array p of a predecessor for each vertex in Q. For example, if a vertex ui in Q can be reached from uj, uk, and ul, and the costs are such that reaching node ui from uk is the least costly, then the predecessor vertex of ui is uk.
Set S starts empty, and in each of the algorithm's iterations one vertex is moved from Q to S. The vertex that is chosen is the vertex with lowest cost from the source node.
The algorithm is:
While Q is not empty:                1. Sort the vertices in Q according to the current best estimate of their distance from the source,        2. Select the vertex u in Q with the lowest cost, and add it to S (and delete it from Q),        3. Relax all the vertices still in Q connected to u; that is, compute the cost of extending the path from u to all nodes that are directly reachable from u, and return to step 1.        
It is noted that step 1 in the above algorithm sorts all vertices in Q and step 2 chooses the vertex with the smallest cost.
It is quite clear that the running time for finding the lowest cost path from one vertex being the source vertex to another vertex being the destination vertex can vary from one situation to another based on the particular costs that are extant. It can be shown that the Djikstra algorithm has a worst case running time on the order of V2. Since the Djikstra algorithm operates by expanding shortest paths around the source node, worst case performance is common, and for large networks, this can involve significant processing times.